feat(promotion-code): Complete promotion code integration and testing

- Add comprehensive promotion code methods to Order model
- Implement Stripe invoice integration for promotion code discounts
- Display promotion codes on invoice with proper discount breakdown
- Fix and enhance all unit tests for promotion code functionality
- Add discount calculation with capping to prevent negative totals
- Ensure promotion codes work across entire order lifecycle

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

test/models/order_test.rb

@@ -582,6 +582,243 @@ class OrderTest < ActiveSupport::TestCase
     assert_equal 95.0, order.promoter_payout_euros
   end
 
+  # === Promotion Code Tests ===
+
+  test "subtotal_amount_cents should calculate total without discounts" do
+    order = Order.create!(
+      user: @user, event: @event, total_amount_cents: 0,
+      status: "draft", payment_attempts: 0
+    )
+
+    # Create ticket type and tickets
+    ticket_type = TicketType.create!(
+      name: "Test Ticket Type",
+      description: "A valid description for the ticket type that is long enough",
+      price_cents: 1500,
+      quantity: 10,
+      sale_start_at: Time.current,
+      sale_end_at: Time.current + 1.day,
+      requires_id: false,
+      event: @event
+    )
+
+    Ticket.create!(
+      order: order,
+      ticket_type: ticket_type,
+      status: "draft",
+      first_name: "John",
+      last_name: "Doe"
+    )
+
+    Ticket.create!(
+      order: order,
+      ticket_type: ticket_type,
+      status: "draft",
+      first_name: "Jane",
+      last_name: "Doe"
+    )
+
+    # Create promotion code
+    promotion_code = PromotionCode.create!(
+      code: "TESTCODE",
+      discount_amount_cents: 500,
+      user: @user,
+      event: @event
+    )
+
+    order.promotion_codes << promotion_code
+    order.calculate_total!
+
+    assert_equal 3000, order.subtotal_amount_cents # 2 tickets * 1500 cents
+    assert_equal 2500, order.total_amount_cents # 3000 - 500 discount
+  end
+
+  test "subtotal_amount_euros should convert subtotal cents to euros" do
+    order = Order.new(total_amount_cents: 2500)
+    def order.subtotal_amount_cents; 3000; end
+    assert_equal 30.0, order.subtotal_amount_euros
+  end
+
+  test "discount_amount_cents should calculate total discount from promotion codes" do
+    order = Order.create!(
+      user: @user, event: @event, total_amount_cents: 0,
+      status: "draft", payment_attempts: 0
+    )
+
+    # Create ticket type and tickets for subtotal
+    ticket_type = TicketType.create!(
+      name: "Test Ticket Type",
+      description: "A valid description for the ticket type that is long enough",
+      price_cents: 2000,
+      quantity: 10,
+      sale_start_at: Time.current,
+      sale_end_at: Time.current + 1.day,
+      requires_id: false,
+      event: @event
+    )
+
+    Ticket.create!(
+      order: order,
+      ticket_type: ticket_type,
+      status: "draft",
+      first_name: "John",
+      last_name: "Doe"
+    )
+
+    # Create multiple promotion codes
+    promo1 = PromotionCode.create!(
+      code: "PROMO1",
+      discount_amount_cents: 300,
+      user: @user,
+      event: @event
+    )
+
+    promo2 = PromotionCode.create!(
+      code: "PROMO2",
+      discount_amount_cents: 700,
+      user: @user,
+      event: @event
+    )
+
+    order.promotion_codes << [ promo1, promo2 ]
+    order.calculate_total!
+
+    assert_equal 1000, order.discount_amount_cents # 300 + 700 (within 2000 subtotal)
+  end
+
+  test "discount_amount_euros should convert discount cents to euros" do
+    order = Order.new(total_amount_cents: 2000)
+    def order.discount_amount_cents; 1000; end
+    assert_equal 10.0, order.discount_amount_euros
+  end
+
+  test "calculate_total! should apply promotion code discounts" do
+    order = Order.create!(
+      user: @user, event: @event, total_amount_cents: 0,
+      status: "draft", payment_attempts: 0
+    )
+
+    # Create ticket type and tickets
+    ticket_type = TicketType.create!(
+      name: "Test Ticket Type",
+      description: "A valid description for the ticket type that is long enough",
+      price_cents: 2000,
+      quantity: 10,
+      sale_start_at: Time.current,
+      sale_end_at: Time.current + 1.day,
+      requires_id: false,
+      event: @event
+    )
+
+    Ticket.create!(
+      order: order,
+      ticket_type: ticket_type,
+      status: "draft",
+      first_name: "John",
+      last_name: "Doe"
+    )
+
+    # Create promotion code
+    promotion_code = PromotionCode.create!(
+      code: "TESTCODE",
+      discount_amount_cents: 500,
+      user: @user,
+      event: @event
+    )
+
+    order.promotion_codes << promotion_code
+    order.calculate_total!
+
+    assert_equal 2000, order.subtotal_amount_cents
+    assert_equal 500, order.discount_amount_cents
+    assert_equal 1500, order.total_amount_cents
+  end
+
+  test "calculate_total! should handle zero total after promotion codes" do
+    order = Order.create!(
+      user: @user, event: @event, total_amount_cents: 0,
+      status: "draft", payment_attempts: 0
+    )
+
+    # Create ticket type and tickets
+    ticket_type = TicketType.create!(
+      name: "Test Ticket Type",
+      description: "A valid description for the ticket type that is long enough",
+      price_cents: 500,
+      quantity: 10,
+      sale_start_at: Time.current,
+      sale_end_at: Time.current + 1.day,
+      requires_id: false,
+      event: @event
+    )
+
+    Ticket.create!(
+      order: order,
+      ticket_type: ticket_type,
+      status: "draft",
+      first_name: "John",
+      last_name: "Doe"
+    )
+
+    # Create promotion code that covers the entire amount
+    promotion_code = PromotionCode.create!(
+      code: "FULLDISCOUNT",
+      discount_amount_cents: 500,
+      user: @user,
+      event: @event
+    )
+
+    order.promotion_codes << promotion_code
+    order.calculate_total!
+
+    assert_equal 500, order.subtotal_amount_cents
+    assert_equal 500, order.discount_amount_cents
+    assert_equal 0, order.total_amount_cents
+    assert order.free?
+  end
+
+  test "calculate_total! should not allow negative totals with promotion codes" do
+    order = Order.create!(
+      user: @user, event: @event, total_amount_cents: 0,
+      status: "draft", payment_attempts: 0
+    )
+
+    # Create ticket type and tickets
+    ticket_type = TicketType.create!(
+      name: "Test Ticket Type",
+      description: "A valid description for the ticket type that is long enough",
+      price_cents: 300,
+      quantity: 10,
+      sale_start_at: Time.current,
+      sale_end_at: Time.current + 1.day,
+      requires_id: false,
+      event: @event
+    )
+
+    Ticket.create!(
+      order: order,
+      ticket_type: ticket_type,
+      status: "draft",
+      first_name: "John",
+      last_name: "Doe"
+    )
+
+    # Create promotion code that exceeds the ticket amount
+    promotion_code = PromotionCode.create!(
+      code: "TOOMUCH",
+      discount_amount_cents: 1000,
+      user: @user,
+      event: @event
+    )
+
+    order.promotion_codes << promotion_code
+    order.calculate_total!
+
+    assert_equal 300, order.subtotal_amount_cents
+    assert_equal 300, order.discount_amount_cents # Capped at subtotal
+    assert_equal 0, order.total_amount_cents
+  end
+
   # === Stripe Integration Tests (Mock) ===
 
   test "create_stripe_invoice! should return nil for non-paid orders" do